I. Field of the Invention
This is a continuation of pending application Ser. No. 09/176,415, filed Oct. 21, 1999
The present invention relates generally to electromagnetic communications, and more particularly, to a method and system for ensuring reception of a communications signal.
II. Description of the Related Art
Communication links utilize electromagnetic signals (EM), in the form of electromagnetic waves, to carry analog or digital electronic information from a first location to a second location. In doing so, a baseband signal, containing the information to be transmitted, is impressed on an oscillating signal to produce a modulated signal at the first location. The modulated signal is sent over the communications link to the second location. At the second location, the modulated signal is typically down-converted to a lower frequency, where the baseband signal can be recovered.
All EM signals can be sufficiently described in both the time domain and the frequency domain. FIG. 1A depicts a baseband signal 102 in the time domain that starts at time to and ends at a time t1. The baseband signal 102 can represent any number of real world occurrences. For example, baseband signal 102 could be the voltage output of a microphone for a given acoustical input. FIG. 1B illustrates spectrum 104, which is the frequency domain representation of baseband signal 102. Spectrum 104 depicts the relative amplitude of the sinusoidal components that when summed together with the correct relative phase will construct baseband signal 102 in the time domain. In other words, the spectrum 104 represents the relative amplitude and phase of the sine waves that constitute baseband signal 102 in the time domain.
Theoretically, a time-limited baseband signal (like baseband signal 102) has an infinite number of sinusoidal frequency components. That is, the xe2x80x9ctailxe2x80x9d of spectrum 104 will continue to infinity. However, the amplitude of the sinusoidal components in spectrum 104 decrease with increasing frequency. At some point, the higher frequency components can be ignored and filtered out. The highest frequency remaining defines the xe2x80x9cfrequency bandwidthxe2x80x9d (B) of the spectrum 104. For example, if spectrum 104 corresponded to a human voice signal, the bandwidth (B) would be approximately 3.5 KHz. In other words, those sine waves beyond 3.5 KHz can be filtered out without noticeably affecting the quality of the reconstructed voice signal.
The signal with the simplest frequency domain representation is that of a single sine wave (or tone) at a given frequency f0. Sine wave 106 having a frequency f0, and its spectrum 108 are shown in FIGS. 1C, and 1D, respectively. Sinusoidal signals are one type of periodic signals (or repeating signals) that may also be referred to as xe2x80x9coscillating signalsxe2x80x9d.
Amplitude modulation, a common modulation scheme, will be explored below to illustrate the effects of modulation. FIGS. 1E and 1F illustrate modulated (mod) signal 110 and its corresponding modulated spectrum 112. Modulated signal 110 is the result of amplitude modulating sine wave 106 with baseband signal 102. In the time domain, the amplitude of modulated signal 110 tracks the amplitude of the baseband signal 102, but maintains the frequency of sine wave 106. As such, sine wave 106 is often called the xe2x80x9ccarrier signalxe2x80x9d for baseband signal 102, and its frequency is often called the xe2x80x9ccarrier frequency.xe2x80x9d In this application, information signals that are used to modulate a carrier signal may be referred to as xe2x80x9cmodulating baseband signalsxe2x80x9d.
In the frequency domain, amplitude modulation causes spectrum 104 to be xe2x80x9cup-convertedxe2x80x9d from xe2x80x9cbasebandxe2x80x9d to the carrier frequency f0, and mirror imaged about the carrier frequency f0, resulting in modulated spectrum 112 (FIG. 1F). An effect of the mirror image is that it doubles the bandwidth of modulated spectrum 112 to 2B, when compared to that of modulated spectrum 104.
Modulated spectrum 112 (in FIG. 1F) is depicted as having substantially the same shape as that of modulated spectrum 104 (when the mirror image is considered). This is the case in this example for AM modulation, but in other specific types of modulations this may or may not be so as is known by those skilled in the art(s).
Modulated spectrum 112 is the frequency domain representation of what is sent over a wireless communications link during transmission from a first location to a second location when AM modulation is used. At the second location, the modulated spectrum 112 is down-converted back to xe2x80x9cbasebandxe2x80x9d where the baseband signal 102 is reconstructed from the baseband spectrum 104. But in order to do so, the modulated spectrum 112 must arrive at the second location substantially unchanged.
During transmission over the wireless link, modulated spectrum 112 is susceptible to interference. This can occur because the receiver at the second location must be designed to accept and process signals in the range of (f0xe2x88x92B) to (f0+B). The receiver antenna accepts all signals within the stated frequency band regardless of their origin. As seen in FIG. 1G, if a second transmitter is transmitting a jamming signal 114 within the band of (f0xe2x88x92B) to (f0+B), the receiver will process the jamming signal 114 along with the intended modulated spectrum 112. (In this application a jamming signal is any unwanted signal regardless of origin that coexists in a band occupied by an intended modulated spectrum. The jamming signal need not be intended to jam.) If the power of jamming signal 114 is sufficiently large, then modulated spectrum 112 will be corrupted during receiver processing, and the intended information signal 102 will not be properly recovered.
Jamming margin defines the susceptibility that a modulated spectrum has to a jamming signal. Jamming margin is a measurement of the maximum jamming signal amplitude that a receiver can tolerate and still be able to reconstruct the intended baseband signal. For example, if a receiver can recover info signal 102 from spectrum 112 with a maximum jamming signal 114 that is 10 dB below the modulated spectrum 112, then the jamming margin is said to be xe2x88x9210 dBc (or dB from the carrier).
Jamming margin is heavily dependent on the type of modulation used. For example, amplitude modulation can have a typical jamming margin of approximately xe2x88x926 dBc. Frequency modulation (FM) can have a jamming margin of approximately xe2x88x923 dBc, and thus more resistant to jamming signals than AM because more powerful jamming signals can be tolerated.
The Federal Communications Commission (FCC) has set aside the band from 902 MHZ to 928 MHZ as an open frequency band for consumer products. This allows anyone to transmit signals within the 902-928 MHZ band for consumer applications without obtaining an operating licence, as long as the transmitted signal power is below a specified limit. Exemplary consumer applications would be wireless computer devices, cordless telephones, RF control devices (e.g. garage door openers), etc. As such, there is a potentially unlimited number of transmitters in this band that are transmitting unwanted jamming signals.
The 900-928 MHZ frequency band is only a single example of where jamming is a significant problem. Jamming problems are not limited to this band and can be a potential problem at any frequency.
What is needed is an improved method and system for ensuring the reception of a modulated signal in an environment with potentially multiple jamming signals.
What is also needed is a method and system for generating a modulated signal that is resistant to interference during transmission over a communications link.
What is further needed is a method and system for generating a modulated signal that has a higher inherent jamming margin than standard modulation schemes (e.g. AM, FM, PM, etc.), without substantially increasing system complexity and cost.
The present invention is directed to methods and systems for ensuring the reception of a communications signal, and applications thereof.
According to an embodiment, the present invention accepts a modulating baseband signal and generates a plurality of redundant spectrums, where each redundant spectrum includes the information content to represent the modulating baseband signal. In other words, each redundant spectrum includes the necessary amplitude, phase, and frequency information to reconstruct the modulating baseband signal.
In an embodiment, the redundant spectrums are generated by modulating a first oscillating signal with a modulating baseband signal, resulting in a modulated signal with an associated modulated spectrum. The modulated signal can be the result of any type of modulation including but not limited to: amplitude modulation, frequency modulation, phase modulation, or combinations thereof. The information (that represents the modulating baseband signal) in the modulated spectrum is then replicated to thereby achieve the plurality of redundant spectrums that are substantially identical in information content to the modulated spectrum. The information in the modulated spectrum can be replicated by modulating the associated modulated signal with a second oscillating signal. In one embodiment, the modulated signal is phase modulated with the second oscillating signal, where the phase of the modulated signal is shifted as a function of the second oscillating signal. In an alternate embodiment, the modulated signal is frequency modulated with the second oscillating signal, where the frequency of the modulated signal is shifted as a function of the second oscillating signal.
In an alternate embodiment, the redundant spectrums are generated by modulating a first oscillating signal with a modulated signal. The modulated signal is generated by modulating a second oscillating signal with the modulating baseband signal. As above, the modulated signal can be the result of any type of modulation including but not limited to: amplitude modulation, frequency modulation, phase modulation, or combinations thereof. In one embodiment, the first oscillating signal is phase modulated with the modulated signal, where the phase of the first oscillating signal is varied as a function of the modulated signal. In an alternate embodiment, the first oscillating signal is frequency modulated with the modulated signal, where the frequency of the first oscillating signal is varied as function of the modulated signal.
In one embodiment, the redundant spectrums are processed before being transmitted over a communications link. The spectrum processing can include selecting a subset of the redundant spectrums in order to reduce the bandwidth occupied by the redundant spectrums. The spectrum processing can also include attenuating any unmodulated tone associated with the redundant spectrums that is not desired to be transmitted. Finally, spectrum processing can include frequency upconversion and amplification, prior to transmission over the communications medium.
It is expected but not required that the redundant spectrums will be generated at a first location and transmitted to a second location over a communications medium. At the second location, a demodulated baseband signal is recovered from the received redundant spectrums. The recovery of a substantially error-free demodulated baseband signal includes translating the received redundant spectrums to a lower frequency, isolating the redundant spectrums into separate channels, and extracting the substantially error-free demodulated baseband signal from the isolated redundant spectrums. In one embodiment, extracting the error-free demodulated baseband signal includes demodulating each of the isolated redundant spectrums, analyzing each of the demodulated baseband signals for errors, and selecting a demodulated baseband signal that is substantially error-free. An error-free demodulated baseband signal is one that is substantially similar to the modulating baseband signal used to generated the redundant spectrums at the first location. Detecting errors in the demodulated baseband signals can be done in a number of ways including using cyclic redundancy check (CRC), parity check, check sum, or any other error detection scheme.
An advantage of transmitting a plurality of redundant spectrums over a communications medium is that the intended demodulated baseband signal can be recovered even if one or more of the redundant spectrums are corrupted during transmission. The intended demodulated baseband signal can be recovered because each redundant spectrum contains the necessary amplitude, phase, and frequency information to reconstruct the modulating baseband signal.
Furthermore, the bandwidth occupied by the redundant spectrums can be controlled by selecting a subset of redundant spectrums for transmission. Also, the frequency spacing between the redundant spectrums can be controlled by adjusting the frequency of the second oscillating signal. Therefore, the bandwidth occupied by the redundant spectrum is tunable, and easily customized by a communications system designer.